Fix acct type signature difference for AIX

[dalvescb]

What does this PR do

Fixes acct mod to provide custom methods that account for differences in AIX's libc::acctl type interface

Checklist:

• I have read CONTRIBUTING.md
• Does not require a change in tests or rustdoc comments
• Does not require to change to nix' API

[asomers]

Why does AIX need a mutable pointer? If there is any possibility that it may modify the caller-provided string, then your pointer cast is invalid.

[dalvescb]

Why does AIX need a mutable pointer?

This is just how AIX defines acct in its system headers (see https://public.dhe.ibm.com/aix/freeSoftware/aixtoolbox/docs/apidiffs.html#read). On linux the function interface is int acct(const char *filename); and on AIX int acct (char *Path); The libc crate requires rust accurately reflects the system headers, hence when we ported it we needed to maintain a mutable pointer

If there is any possibility that it may modify the caller-provided string, then your pointer cast is invalid.

Despite the difference in interface, the Linux and AIX definitions of acct are documented as compatible and acct does not modify its argument. So the cast should be valid